A quark is an elementary particle with fractional electric charge and Spin 1/2. All quarks are therefore fermions. Quarks are the fundamental unit with which hadrons are composed of and are one of the major components of the Standard Model. Being fermions, they are described by the Dirac equation, which implies the existence of antiquarks. There are six flavors of quarks:
Baryons are particles made of three quarks or antiquarks and mesons are made of one quark and one antiquark. The theoretical description of quarks as a whole is given by the quark model.
Quarks are known to interact with all four fundamental interactions. They are an integral part of the strong force, as quarks appear to be the only particles in existence capable of interacting this way (barring, of course, the gluon gauge boson that is the force carrier).
Etymology#
The name "quark" comes from a nonsensical passage in James Joyce's Finnegan's Wake:
Three quarks for Muster Mark!
Sure he hasn't got much of a bark
And sure any he has it's all beside the mark.
Murray Gell-Mann, a fan of Joyce, decided to take the word to name these particles.